I had this intermittent problem with a flashing padlock icon, and the On Screen Menu would on occasion pop up as well. I understand the lock/unlock options on the monitor buttons, but since I wasn't pressing a button, and it would randomly flash, this couldn't be the problem. It was acting like I was pressing a button.
I did some reading at various locations which suggested changing the resolution and changing it back, locking and unlocking by holding the menu button down for 15 seconds, etc. These do not work.
What is going on is that the contacts on the small circuit board behind the buttons have become dirty or wet. The solution is to clean them. Most dell monitors have a front bezel that pops off.
1. I would take the monitor off the stand, disconnect the cables, and place it on it's back.
2. Put something flat behind the dell logo (even fingernails will work) and slowly pop the bezel up all the way around.
3. There should be a small circuit board with a ribbon cable attached where the buttons are. There should be some really small screws holding the circuit board to the bezel. Remove the screws.
4. Take a cloth, and clean the back of the buttons on the bezel, and clean the contacts on the circuit board.
5. Re-assemble in reverse order.
This 100% fixed my flashing padlock problem.
Interesting fix. But, Dell could only condone this for out of warranty monitors. If an in warranty customer attempted this and broke something, Dell would not cover it and the customer would lose whatever purchased warranty was left.
This fixed the problem for me on my U2715HC. Kids playing on the computer, that part got fairly dirty. I took a paperclip and shoved a piece of toilet paper down behind the buttons and swished it around. Problem solved!